Conventionally, coating is applied to metal can materials, such as tin free steels (TFSs) and aluminium, used for food cans. However, the technique employing such coating application has problems as it not only involves a complicated baking step, but also requires much processing time, and further, discharges much solvent. To solve these problems, many techniques have been proposed in which a thermo-plastic resin film is laminated over a heated metal sheet.
Many of the proposals relate to improvement in adhesion property and formability of a film and a metal sheet used as a base material. The proposed techniques can be briefed such that the technical concepts thereof concern: (1) use of a film (such as a polyester resin film) having a polar group (as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 63-236640); and (2) an increase in surface free energy represented by, for example, activation by processing such as the process of corona discharge to a film surface (as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 05-200961). Japanese Unexamined Patent Application Publication No. 05-200961 discloses in detail that the film-surface free energy is controlled to a range of (38 to 54)×10−3 N/m (38 to 54 dyn/cm) to secure, for example, post-fabrication adhesion property of a polyethylene-resin coated metal sheet.
However, in the technique using the laminate metal sheet proposed as in the above-referenced publication for food cans, when attempting to release a content substance from the container, a problem takes place in that since the content substance strongly adheres to the inner surface of the container, the content substance cannot easily be released. This problem relates to purchase motivation of consumers. As such, improving releasability of the content substance is very important. However, conventionally, no considerations have been made to improve the releasability of the content substance.